Multimode fiber optic backbone cables are being deployed to connect sections of high-speed networks together, and for long distance communications. To secure these high-speed networks, software based Intrusion Detection Systems (IDSs) have been introduced. These systems capture and analyze all packets for unusual patterns that point to an intrusion as well as monitor systems accessing a network. However, this adds to the complexity of the network and burdens processing power. Current IDSs are hampered by Base-Rate Fallacy limitation, which is the inability to suppress false alarms. Additionally, software-based IDSs do not provide protection against passive optical fiber tapping, which can go undetected by the network hardware. Software IDS is the de-facto standard for intrusion detection, however it is oblivious to actual physical layer intrusion and perturbation such as tapping or the attendant fiber handling.
It is well known, by those skilled in the technology, that optical fibers are easily tapped and the data stream intercepted. One relatively simple non-interruptive tapping method involves placing a bend coupler on the fiber to be tapped. A controlled bend of a critical radius is placed on the fiber. This causes a small spatial distortion in the core/cladding guiding properties and a fraction of the light escapes the fiber. A detector is located at the point of the light leakage and the data steam intercepted. Bend couplers typically introduce a loss of light power of up to 1 dB or more. Power measuring intrusion detection systems are available to detect this loss in optical power and provide warning alarms.
With care and skill, more insidious methods are available to the skilled intruder. With a sufficiently sensitive receiver and care in preparation, a fiber can be successfully tapped without introducing a telltale bend in the optical fiber. A successful tap can be achieved by carefully removing a few inches of the protective outer coating of the target fiber and polishing, etching, or otherwise reducing the outer cladding down by a few microns to form a flat coupling region. A cladding-to-cladding coupling is then made using a special intercept fiber. This method intercepts a portion of the weak but measurable evanescent power that propagates along the tapped fiber. In this case, the intercepted light, which is detected by a sensitive receiver, can easily be 20 or 30 dB down from the power in the fiber core. This results in a loss of received optical power of only 0.04 or 0.004 dB and is impossible to detect reliably by power measurement methods.